The conventional LPS or endotoxin preparations are heterogenous. Besides endotoxic components they contain non-toxic substances which are active in some of the beneficial effects attributed to endotoxins. These effects are immunopotentiation, enhanced proliferation of bone-marrow cells, resistance to infections, activation of the RES and increased resistance to a few experimental tumors. We found that this beneficial component (B component) of the commonly used LPS or ET preparation can be separated by chromatographic methods from the toxic component. Furthermore, we found that such B component can also be extracted directly from some rough mutant bacteria, from which the endotoxic components were previously removed. The aim of this work is the characterization and chemical structural studies of the B component.